Did you ever wonder how researchers determine levels of confidence? Typically this research is perform using a population of college students volunteer to take part in psychological tests (sometimes a requirement for a psychology course). To measure confidence, students take a general knowledge test and estimate how well they did. Most of the students are not very accurate at guessing their performance on the tests. Typically 40-50% typically are over confident, and 30 to 40 % are under confident, and the small remaining percentage accurately estimate how well they have answered the questions.
Of course the whole purpose of the test is not to find out how many over confident students exist, but to find the correlation between over or under confidence and some other trait. Sometimes populations are chosen based on this other trait, such as research on gender differences in grades for the same major. These studies reveal that men on the average are over confident; while women o are under confident. 
Often research includes additional personality assessments tests to determine personality traits that lead to overconfidence. Assessments based on the five factor model (commonly called the “Big Five”) are used with great frequency. These five factors are: openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. The acronym OCEAN is used to make them easy to remember.
According to research combining the test of confidence and a five factor analysis, high scores in extraversion significantly predicted overconfidence. Of course, that doesn’t mean that all extroverts are over confident and introverts are under confident, However, a greater chance exists of these traits appearing together, even though it is a small one. There is also a mild correlation between agreeableness and overconfidence. The people who rank higher in both extraversion and agreeableness showed the most significant correlation to being over confident compared to others. 
What you must realize first is that the five factor analysis is almost always a self-reported test. It is not surprising to see extraversion correlated with over-confidence, because the five factor analysis is almost always a self-reported test.. Extraverts tend to measure themselves higher in confidence, In actuality, their scores were lower than they predicted on tests of general knowledge.  Their inaccuracy tended to be biased to make them look better rather than make them look worse.
So what does agreeableness have to do with being overconfident? It would seem odd that students that characterize themselves as being friendly, cooperative, good-natured, sociable and nurturing would also inflate reports of their abilities. Agreeableness is seen as a positive characteristics. Perhaps people who want to appear better to others grade themselves higher in the area of agreeableness, even if they aren’t as agreeable as the students sitting next to them. Perhaps they even believed their own inaccurate report of themselves. It is easy to do when the rest of us take these self-reported qualities at face value without examining them.